This invention relates to an adapter which permits a temperature measuring instrument designed only for use with a particular type of temperature sensor to be used with temperature sensing probes of a type having a temperature sensing element different from that of the temperature sensor. More particularly for example, the present invention relates to an improved adapter for producing a resistance which simulates a thermistor in response to the output voltage of a thermocouple. The invention allows the use of thermocouple sensing probes with a temperature measuring instrument designed to utilize a thermistor temperature sensor. This invention also relates to an energy efficient temperature measuring instrument powered by a battery.
For temperature measurement of a patient during surgery and for other thermometric applications, thermocouples are widely used. These are formed from a tiny sensing junction between the ends of two thermally conductive fine metallic wires such as copper and constantan and a reference thermojunction of like metals. The thermocouple produces a voltage which is a function of the difference between the temperatures of the sensing and reference junction. Because of its small size the sensing thermojunction responds rapidly to changes in the temperature of the patient, can be conveniently applied to any selected point on or in the patient's body and compared to thermistors, thermojunctions are less expensive, disposable and have less thermal "mass" for faster temperature response.
Many presently available temperature monitoring and display instruments are designed for use only with thermistors. The owners of this equipment would prefer to be able to use disposable thermojunction sensors, but are deterred by the cost of replacing such instruments with those designed for use with thermocouples. It would be desirable therefore to be able to utilize disposable thermojunction sensors with these existing instruments intended for use with only thermistor sensing elements.
The patient whose temperature is to be measured must be safe from electrical shock. To provide electrical isolation to protect the patient from electrical shock, battery energized temperature measuring instruments are often used. But unless energy consumption is minimized, the batteries will have to be replaced frequently.
One approach to an adapter intended to interface thermojunctions with temperature monitoring and display instruments using thermistor sensors is shown in Lindberg, U.S. Ser. No. 312,010 filed Oct. 16, 1981 now abandoned of which the present assignee is also the assignee. Briefly, Lindberg placed a thermistor, having its terminals connected to the output terminals of the adapter, in conductive heat transfer relationship with an electrical resistance heater. At input terminals of the adapter was connected a thermocouple which produced a voltage functionally related to the temperature and used by a control circuit. The control circuit caused the electrical resistance heater to be heated so that the temperature of the thermistor would be at the temperature corresponding to the thermocouple voltage thereby having the thermistor produce the corresponding resistance. This adapter had some inherent disadvantages. Precision heating with the electrical resistance heater and rapid dissipation of heat with a falling temperature is difficult to obtain. Because the thermistor was incorporated in heat transfer relationship with the electrical resistance heater, the adapter could not be conveniently used with different types of temperature measuring instruments having inputs requiring different thermistor characteristics. The adapter also required substantial amounts of energy from a battery which necessitated its frequent replacement.